This invention relates to an ink-jet printer with at least page-wide printhead structures and especially to a system for aligning these printhead structures with respect to each other and the image receiving substrate.
Ink-jet printing has become a widely used printing technique especially in the digitally controlled electronic printing business.
Many types of ink-jet printing mechanisms have been invented. These can be categorised as either continuous inkjet (CIJ) or drop on demand (DOD) ink-jet. Using one of these type of ink-jet printing, colour printers have been designed, wherein from multiple printhead structures different colours are printed. Properly controlling the arrangement of various droplets of ink of different colours will result in a wide spectrum of perceivable colours. The clarity and quality of the resultant image is affected by the accuracy of the placement of the ink droplets on the medium. Printers which use multiple printhead structures to co-operatively form a single image usually require mechanical or electronic adjustment so that ink droplets printed by one printhead alight at precise locations on the receiving medium relative to those printed by another printhead in the printer. Several methods to achieve the accurate alignment of the rows of droplets ejected by the different printhead structures have been proposed.
For example, in U.S. Pat. No. 5,600,350 titled Multiple Inkjet Print Cartridge Alignment By Scanning A Reference Pattern And Sampling Same With Reference To A Position Encoder, U.S. Pat. No. 5,448,269 titled Multiple Inkjet Print Cartridge Alignment For Bi-directional Printing By Scanning A Reference Pattern, U.S. Pat. No. 5,451,990 titled Reference Pattern For Use In Aligning Multiple Inkjet Cartridge, U.S. Pat. No. 5,404,020 titled Phase Plate Design For Aligning Multiple Inkjet Cartridges By Scanning A Reference Pattern, U.S. Pat. No. 5,350,929 titled Alignment System For Multiple Colour Pen Cartridges, U.S. Pat. No. 5,297,017 titled Print Cartridge Alignment In Paper Axis, and U.S. Pat. No. 5,250,956 titled Print Cartridge Bi-directional Alignment
In U.S. Pat. No. 5,534,895 the ink-jet printer is equipped with a source of illumination that is passed across a test pattern having features indicative of printhead structure alignment and discernible under the illumination. The source of illumination is connected to circuitry that determines the variation in light intensity of the test pattern. A value indicative of the misalignment is calculated and used to correct the timing of firing signals between the sequentially fired banks of nozzles of a printbar.
In U.S. Pat. No. 5,751,305 it is disclosed to place a referencing mechanism on the printer and a detector on the printhead in order to dynamically align one or more printheads in a printer. The printhead structure is moved at a known speed past two spaced apart reference indicia of the referencing mechanism. The passing of a first of the spaced apart reference indicia is detected and the passing of a second of the spaced apart reference indicia is detected. The time between the detection of the first reference indicia passage and the detection of the second reference indicia passage is measured and a delay time, related to the measured period of time, is created. Energization of an ink drop ejection is delayed for the duration of the delay time.
In U.S. Pat. No. 5,192,959 an alignment system for a pagewide printhead structure is disclosed. The pagewidth printhead structure would include a reference plate, a linear array of ink jet sub-units affixed to the reference plate, and a plurality of alignment sub-units affixed on opposite ends of the planar surface of said reference plate. The ink jet printer would also include alignment or reference points for engaging the alignment sub-units and thereby aligning the pagewidth printhead structure with respect to the frame. However once the printhead structure is aligned in the frame no further fine tuning of the alignment is foreseen.
In U.S. Pat. No. 6,109,721 a bi-directional print position alignment system for automatically aligning bi-directional printing position of a printhead structure in a serial printer as a function of high sensor accuracy and clock frequency of a CPU controlling the sensor. The alignment system includes a sensing section for sensing a position of a printhead structure for vertical alignment, a misalignment detecting section for detecting mechanical misalignment of the printhead structure, and a printing section for correcting said mechanical misalignment of the printhead structure and printing information on a printable medium after said mechanical misalignment of the printhead structure is corrected.
In U.S. Pat. No. 6,109,722 and U.S. Pat. No. 6,076,915 test patterns are disclosed that are useful for printhead structure alignment. The test patterns are optically sensed and the sensed pattern are used to electronically adjust the alignment, either by adjusting the firing time of the nozzles, either by shifting the pattern of ink-jet nozzles from which the ink is ejected.
Although the teachings of the prior art do allow for a good alignment of printhead structures, it is still desired to have a system for printhead structure alignment that makes it possible to align in more than one direction and/or over a fraction of the nozzle pitch.
The present invention is a method for aligning printhead structures in an ink-jet printer as claimed in independent claim 7, and a system in which the method is implemented as claimed in independent claim 1. Preferred embodiments of the invention are set out in the dependent claims.
Advantages and further embodiments of the present invention will become apparent from the following description and drawings.